Designing a Cellular Mobile Radio Communication System for Gold Coast, Australia - Research Paper Example

Designing a Cellular Mobile Radio Communication System for Gold Coast, Australia Abstract This paper contains a design for a small cellular mobile radio system for Gold Coast, Australia. The operating frequency of the cellular system is in the range of 900 – 2200 MHz. The coverage of the cellular mobile radio system takes into account the cell coverage for signal and traffic, cell-site antennas and mobile antenna, cochannel interference reduction as factors and components involved in the design. The design also includes large-scale path loss and small-scale fading into consideration along with multipath has been presented. 1. Introduction The past couple of decades witnessed, the evolution of mobile communications from a narrow and specialised field with limited interests to one of the most important fields of telecommunication [Kun10]1. Consequently, rapid growth is achieved in this sector due to new services and emerging technologies making social and business activities to rely heavily on wireless communications [Dav105]2. Mobility, coupled with unbridled access to resources that were earlier domains of fixed infrastructure networks e.g., the public switched telecommunication (PSTN) and the Internet etc. is giving users flexibility in new opportunities for efficient use of telecommunication [Kun10]1. 1.1 Background Mobile radio communication is a broad term, which encompasses an array of systems and techniques, provides a variety of combined services such as speech, data, messaging or simply paging, involving a series of such diverse areas as, Telecommunication services, both traditional and mobile specific Radio transmission, i.e. propagation, properties of different frequency bands, antenna design, transmission and reception, modulation, etc. Communication protocols for signalling and user-to-user data transfer Network architecture, system configuration, call routing Network operation and management Source coding, e.g. compression techniques for speech or video signals. Wireless network typically comprise analogue and digital cellular phone networks, personal communication systems (PCS), wireless local area networks and wide-area mobile data service networks such as general packet radio service in GSM systems[Dav105]2. Of these, these two sectors are experiencing fast growth. These are the mobile cellular and PCS networks with emphasis on providing voice, data and multimedia convergent services to the users. More research is being done to make these services better with higher data rates irrespective of the location, mobility pattern or type of terminal used for access. Overall, despite inferior quality and reduced service offering in comparison to wired networks, the flexibility achieved on account of mobility is making it an indispensible commodity throughout the world[Dav105]2. The primary objective of a cellular system is to provide wireless connectivity in terms of telephonic services to a user located within its service area[Yac96]3. Designing a cellular system is a highly complex process involving knowledge and expertise in many fields such as radio propagation, frequency regulation and planning, modulation, antenna design, transmission planning, switching, teletraffic and software design [Yac96]3. Without an in depth understanding of economic and human factors, the knowledge and expertise of a cellular design engineer is never complete. In addition, the knowledge of cellular technology comprising hexagonal grid, channel assignment, cell splitting overlaying of cells, call processing, and radio propagation principles is also essential[Yac96]3. For undertaking the designing of a cellular system, a phased approach is adopted[WCY]4. The phases include such aspects as estimation of future scenarios, collection and measurement of data and reconfiguration of the system if necessary. Having a fair estimation of the future as to what is likely to happen in 20 years or 30 years from now is an important task of envisioning a physical infrastructure project such as this[Des2]5. Collecting and measuring data is a necessary aspect of any cellular designing. This is to be catered for while the system is operational and continuing to serve its users. This helps in developing the insight of how well the system is able meet the design objectives[Des2]5. The third phase that the designing should cater to is the need for reconfiguration at a later date. In the initial stages of the deployment of the cellular system it is likely that operational achievements fall short of design objectives , in which case undertaking necessary corrective action becomes necessary [Des2]5. All the phases need different level of expertise and techniques of handling. Even as early as 1996 Yacob stated that designing a cellular system requires considerable amount of information needs to be gathered, especially on such matters as, the service area under consideration, definition of traffic profile, choice of reuse pattern, location of the base stations, radio coverage prediction, design check-ups, field measurements and so on [Yac96]3. 1.3 Previous work The previous work in planning and designing of cellular mobile radio communication systems were based on pragmatic approach, devoid of detailed planning methodology and step-by-step network implementation. It primarily meant implementing cell sites at locations usually selected by radio engineers experienced in the field and providing measurements of the running system followed by adopting it. This approach often resulted in over-dimensioning of the system, since it required additional equipment without having removed the needless equipment to use at other cell sites[KTu10]6. However, as customer demand rises to new pitch coupled with competition and consequent falling of customer charges, it becomes necessary to adopt more systematic planning and designing approach. 1.4 Proposed work The present work involves designing a cellular mobile radio communication system using the contemporary analytical approach for the city of Gold Coast, Australia and its suburbs with an operating frequency range of 900 – 2200 MHz. This approach lays emphasis on the radio engineering aspects such as determining the locations of the cell sites and assigning frequencies to them by examining the radio-wave propagation environment and interferences among the cells. [KTu10]6. For achieving optimum capacity utilisation and efficiency of bandwidth spectrum, the frequency reuse factor of seven was chosen. Other aspects such as user behaviour and teletraffic issues were also taken into account. In addition, fundamental mathematical properties of hexagonal cellular geometry have been employed to provide uniform coverage and co-channel interference reduction. Requirements of sectoring and splitting of cells have also been taken into account along with traffic demand, number of handoffs required, characteristics of radio propagation region, signal impairments and provision of antenna systems needed for different cells and sites. In the next section, the theoretical aspect of designing is discussed. 2. Design and Theory 2.1 The Cellular Concept In the early days of the evolution of the mobile communication, it was structured similar to the television broadcasting, with one very powerful transmitter positioned at a very conspicuous location of the service area, with a radius of up to fifty kilometres[The105]7. Figure 1: Early mobile telephone system architecture [The105]7 This had many limitations, which were greatly overcome by the cellular system. In this system, instead of one powerful transmitter, many low-power transmitters were positioned throughout the area required to be brought under the coverage area. Figure 2: Mobile telephone system using a cellular architecture[The105]7 In a cellular mobile radio system, a large number of mobile units are covered within a limited frequency spectrum. The figure below depicts a cellular system comprising mobiles, base stations and a switching centre. The mobile phones communicate through radio frequency with one or more base stations. Thus a call from a one user can be transferred from one base station to another while the call is in progress the process is referred to as handoff[med]8. The area required to be covered is divided into cells with assigned frequency range for each cell, which is a basic geographic unit of a cellular system[The105]7. The cellular concept provided a major breakthrough in finding a solution to the spectral congestion problem, while offering higher system capacity with the same limited allocated spectrum. The chief objective of a cellular system is to transmit at power levels which are high enough to provide satisfactory transmission quality, yet it should be low enough so that it does not cause interference with nearest station in which the same channel is reused, thereby giving scope for reuse in the given geographical location[WCY]4. Figure 3: An illustration of a cellular radio system[med]8 Clusters A cluster comprises a group of cells, where no channels are reused within the cluster[The105]7. In the figure below, a seven-cell cluster is depicted. Figure 4: Illustration of a seven cell cluster [The105]7 2.2 Cellular Principles Frequency reuse, which is the core of cellular concept, implies repeated use of the same set of channels in geographically separated locations assigned sufficiently apart from each other thereby co-channel interference is minimised. In a situation, where a group of cells are assigned with the set of available frequency channels, they form a cluster. The cells are designed in hexagonal shapes with the number of cells contained in a cluster determines the iteration pattern [Yac96]3. However, due to hexagonal shape of the cells, only a few repeat patterns can fit with each other. In a cluster, the number of cells ‘N’ is given by the formula: = (1) where ‘I’ and ‘j’ are integers. Thus From Eq. 1, it is evident that a cluster can accommodate only certain numbers of cells such as 1,3, 4, 7, 12, 13, 19, 21..., the most common being 4 and 7[Yac96]3. The cell capacity and the transmission quality is based upon the number of cells are located in a cluster [Yac96]3. Hence, the lesser are the number of cells assigned in a cluster, the greater would be the number of channels per cell. Conversely, higher is the capacity and the closer the co-cells, the probability of co-channel interference becomes more [Yac96]3. Additionally, in hexagonal geometry, it can be illustrated that the ratio of distance ‘D’ and radius ‘R’, between the co-cells is given by the formula: (2) Further, area ‘A’ covered by a cluster can be calculated by the formula[Des]9, And Co-channel interference ratio CCR is given by the formula Where gamma is the propagation path loss slope and varies between 2 and 5[Des]9. An important aspect is the frequency spectrum, which is a precious commodity due to limited bandwidth availability. Therefore, optimisation techniques such as frequency reuse are employed to ensure best capacity utilisation while minimising interference between cells using the frequency channels[Des]9. Figure 5: The mobile cellular network architecture [Dav105]2 2.2 Frequency division and time division multiple access Frequency division and time division multiple access Frequency division multiple access (FDMA) and time division multiple access (TDMA), these are two access techniques employed to share the available band width in a conventional mobile radio communication system. While FDMA assigns individual channels (frequency bands) to individual users, TDMA divide the transmission time into time slots, and in each slot one user is permitted to transmit or receive[med]8. Figure 6: Illustration of a fixed wireless access [The105]7 2.3 CDMA Three types of CDMA techniques are in use, (1) direct CDMA, Frequency hopped CDMA and pulse position-hopped CDMA. In a CDMA a radically different approach is employed. Here the narrowband message signal is multiplied by a large band-width signal referred to as spreading signal. All the users in a DS CDMA system use the same career frequency and can transmit simultaneously. Thus each user has its own spreading signal, which is orthogonal to the spreading signals of all other users. Here the receiver does a correlation operation to detect the message addressed to a give user. Signals emanating from other users appear as noise as they are not correlated[med]8. 2.4 Radio frequency engineering Radio frequency engineering is based on the idea that the system needs to provide highest possible performance in the radio portion of the system at the same time the cost is kept to the minimum. Radio performance takes both the quality of control transmission path and the quality of the voice transmission path into consideration[WCY]4. Further, radio propagation necessitates a comprehensive consideration of the topography, terrain structure, the urbanisation factor, clutter factor of the city. Therefore, these inputs are essential while designing the radio coverage [Yac96]3. Radio performance includes both the quality of the control transmission path and the quality of the voice transmission path, measured by RF signal-to-impairment ratio, S/(I+N), where N is the noise and I is the co-channel interference and impairment is the power the sum of noise and co-channel interference [Des2]5. Based on several studies which indicated that in the range of a 15- to – 25 db S/(I+N), with relatively small improvements in S/(I+N), customer satisfaction improves considerably, for the purpose of this work, the recommended level of 17 –db S/(I+N) was aimed to be achieved in the service area of each omnidirectional and directional site face. Besides, this level of S/(I+N) is economically feasible[Des2]5. 2.5 Performance Measurement Two parameters are considered while grading the cellular systems: these are carrier-to-cochannel interference ratio and blocking probility. A high carrier-to-co-channel interference ratio in connection with a low-blocking probability is the desirable situation[Yac96]3. For example, this can be achieved in a large cluster with a low traffic condition. In such a situation, the required grade of service can be accomplished, despite that the resources may not optimally utilised. This is the reason why, a measure of efficiency is important. The spectrum efficiency expressed in ‘erlang’ per square meter per hertz, suggests a measure of how efficiently space, frequency and time are used. Spectrum efficiency is given by the formula [Yac96]3. Trunking efficiency is another important measure of interest. The trunking efficiency is the number of subscribers per channel is obtained as a function of the number of channels per cell for different values of blocking probability[Yac96]3. 2.6 Traffic engineering The starting point for engineering the traffic is the knowledge of the required grade of service. This is usually specified to be around 2 percent during the busy hour. Three possible definitions of busy hours are available : (1) busy hour at the businest cell, (2) system busy hour and (3) system overall hours. 2.5. System Expansion Techniques Lee suggests the following important techniques for undertaking expansion of cellular systems. These are (1) by adding new channels, (2) by frequency borrowing, (3) by changing cell patterns, (4) by cell splitting, (5) by sectorisation and (6) channel allocation. 3. Methodology 3.1 Design Considerations The design of a cellular radio mobile is usually undertaken in four phases (1) system configuration, (2) followed by the performance, (3) if the test fails, several diagnostic procedures are adopted to ascertain the reason of failure, (4) and once the causes have been established, modified system reconfiguration is undertaken, which would satisfy the S/(I+N) ratio performance test criteria [Des2]5. This process is repeated until the specified until the required criteria is achieved. The overview of the design process is appended in the figure below. Figure 7: Overview of Engineering Considerations [Des2]5 3.2Configuring the System The system configuration involves developing a cellular grid, with specified site locations, determining transmitter power, antenna types to be used and their heights. Usually while designing a new system as that is presently done for the Gold Coast, all antennas are made omnidirectional. From the coverage area of the cell, their height is determined and the power-outputs of the transmitters are set to the maximum permissible limit. In addition, the voice and setup channel groups are assigned. Subsequently, after an iterated process to support RF coverage into all areas and to support expected traffic, the final configuration is determined. This system of configuration is the best representation of the trade-off between the various performance criteria and cost and practicality [Des2]5. Figure 8: The process of system configuration 3.3 Cell Structure and Cell Sizing The number of channels per cluster (N) is determined Depending on the co-channel interference. The smaller is the value of N, the lower will be the S/I performance. Studies indicate that for smaller systems with less than 1,000 square miles, such as Gold Coast, the start-up configuration can have idealised hexagonal cells with same area. For larger systems, more cell sizes is required, at times even going up to three cell sizes[Des2]5. However, the present study area of 414.3 km², it was decided that the service area be configured based on a single cell radius. For assigning channel groups to cells, the procedure remains same for both omnidirectional and directional sites. Initially, channel groups are assigned by overlaying an array of regular hexagons on a map of the service area, each hexagon containing only one site[Leo93]10. Even though, the hexagons may be of same size, but due to the fact that there are regions of different traffic densities through which radio propagation would be required to traverse, it was decided that smaller cell sizes must be catered. The figure below shows the cell structure for Gold Coast. The circles indicate the increasing population as the suburban areas are covered outwards from the centre of the city. Figure 9: Cell radius indicator for Sunshine Coast Image URL: http://upload.wikimedia.org/wikipedia/commons/thumb/1/19/Gold_Coast_Suburbs_Map.gif/420px-Gold_Coast_Suburbs_Map.gif The region covered under of 0 to5 Km was designed to be served by 3 different cell sizes. Taking Worongary as the centre, cell sizes of 500 m were allocated at the centre of the map and spread outward while increasing the cell size. The magnified view of the 500 m cell radius superimposed on a Wikipedia map of Worongary area with each grid at approximately 500 meters is placed below. The centre most-cell is of the radius 1.5km having six hexagons of same size surrounding it. As the size of the service area is increased moving away from the centre, the cell radius is increased proportionated with the population density and tele-traffic. Next to 1.5 km cell radius, 3 km cell radius is taken until the first circle of 5 km is reached (Figure 9). The figure above illustrates the cell design for the innermost region of the service area under consideration. The size of the cell with radius of 3 km has been made smaller into the one with the size 1.5 km to cater for higher teletraffic in the vicinity of the city centre. As the service area is increased outwardly from the city centre, the region between 5 to 10 km radii (Fig. 9), the population density starts decreasing with consequent decrease in the traffic demand. Accordingly, the cell radius in this region is relaxed to six km. Eventually, as the service area is increased from 10 km to 20 km region of Fig 9, even lesser population density and consequent lesser traffic demand is encountered. Many areas also include coasts, forests and mountains, water bodies, which can be served by a large cell size with one base station in the centre of the cell, keeping the radius of 12 km on the map of Figure 9. 3.4 Reusing frequency and assigning channel group In Fig. 10, the start-up configuration with all the cell sizes is shown. In the beginning a channel group to each cell in large cells is assigned, thus allocating all the largest cells from the map of the wireless system. Then, next smaller size cells are assigned, which uses the completed pattern of assignments for larger cells. A small cell’s channel group is assigned by locating a co-cell to which a channel group has already been assigned earlier. The allocation of channel group for a regular size hexagonal grid pattern is as placed below. Figure 14: This figure shows completion of channel group assignment for large-cell grid [The105]7 When the cell sizes have grown smaller for a particular region, the procedure of assigning channels is undertaken as in Figure. 10. It is to be noted that the small cell on channel-group 6 is mapped to the hypothetical cell on group 6, while group 4 is mapped to group 4. When the allocation is done, the assigned channel group would be as depicted in Figure 8. This completes assigning channel groups. Figure 15: Complete system map with channel-group assignments for large cells [The105]7 Figure 16: method of assigning channel groups to small cells [Des2]5 Once the process of assigning channel groups is complete, the next step is to take decision on the number of channels required for an omnidirectional cell site or for each face of a directional site, and see how the channel sets within the group are to be used[WCY]4 3.5. Propagation and Path Loss analysis for Gold Coast Considering that the characteristics of RF propagation may vary much depending on the terrain, adopting a single value of cell radius will not suffice the requirement. As path loss increases relatively slowly with distance, for the highlands and mountainous region in the west, north and south of Gold Coast, selection of a relatively large radius would be appropriate. Hence a radius of 12w Km is chosen for these regions of Gold Coast. Figure 17: Topographical map of Gold Coast. For the high and mountainous land on the west, north and south larger cell sizes would be appropriate. On the other hand, this would create the problem of deteriorated performance in the more heavily inhabited regions in the east. Therefore, to counter this problem, comparatively smaller sized cell radius is chosen for sections where path loss is severe. Other factors that are required to be taken into account while selecting smaller cell radii for eastern part of Gold Coast would include urbanisation factor, clutter factor of the city, heavy to severe traffic density and large number of handoffs per unit time. 3. 6 Antenna System For the hilly part of Gold Coast on the outer suburbs in the west, directional antennas were proposed with appropriate angle of tilt so that low lying areas are not missed due to line of sight nature of RF waves are not missed. Figure 18: Directional switched beam antenna for hilly regions [Des]9 The figure below represents by suitably positioning a directional antenna on top of a hill, signals from adjoining low lying areas can be picked up. Note the hexagonal cell size on the valley with a base station in the middle. As evident from the Google Map of Coomera region of Gold Coast, the base station is proposed at north of Upper Coomera and the cell radius for the valley area is kept small due to kept small, on the other hand, the two base stations on the hill (yellow dot, and yellow dot encircled in red) will be able serve a relatively larger area. Figure 19: Positioning of a directional antenna on top of a hill to serve low lying areas In the city area of Gold Coast and the central region, omnidirectional smart antenna is proposed due to heavy signal processing requirement from units moving in and out of the region. This would also require catering for handling large number of handoff requirements at a faster rate in comparison to the suburbs of Gold Coast. Smart antennas have the capability to make use of interference due to signals emanating from many paths to create the desired signal even in cases where extreme high level of noise level is experienced. Smaller size cells in the city area also will ensure preventing call drops. 3.7. Designing for Growth In the year 2006, the Gold Coast’s population was 454, 436. The population density of the region is 972/km²; it is the second most populous city in the state and the sixth most populous city in the country.[htt1]11. Gold Coast’s population is increasing at an average annual rate of 2.66 percent, which means that there will be increased requirement for cellular services. This would necessitate the designing to cater for future population growth at least up to 25 years. When the population is sufficiently grown, there may be a requirement for splitting the cells in line with the areas where increased demand is likely to be encountered. 4. Discussion and analysis In the foregoing sections, the planning and design engineering process was discussed for designing a cellular mobile radio communication radio system for Gold Coast. The cell geometry, frequency reuse factor, cell splitting was illustrated. In addition, the way cell structure growth is implemented along with their application for designing the city of Gold Coast and the suburban areas were illustrated. The city of Gold Coast offered many challenges due to its typical topological features characterised by hilly highlands and mountainous terrain towards the west, north and south, at the same time, the land adjoining the eastern seaboard and its immediate vicinity frequently interspersed by canals and waterways. Accordingly, the traffic distribution in the city and the suburbs varied both in time and space depending upon the geographical location of the sites. Towards the eastern coastal lowlands and the flat midlands close to it, where the city also houses its business districts, witnessed during rush hour rapidly decreasing towards the outer fringes. A reversal of the pattern was observed towards the end of the day when the users would be moving back towards their residences in the suburban areas. Due to the movement of users, roaming and handoffs takes place frequently resulting in small channel acquisition times, on the other hand increasing the traffic considerably in the areas where they travelled. Under such dynamic traffic environment, performance of the traffic was given full consideration to determine future growth. In urban areas, where large concentration of the traffic was found, smaller cell size was proposed to cater to meet heavy demand on the channels. In comparison, in the suburban areas, where teletraffic was less, larger cells were proposed. The first base station was proposed at Worongary, located in the central midlands of the city. After defining the first base station, other base stations were located following an iterated process. Besides, the traffic available at the boundary between the cells, those had the requirement to communicate with multiple base stations, were also given due consideration. It was considered that as the teletraffic density became intense at the boundaries, to optimise channel assignment, the possibility of segmentation of the cells at the boundaries would be necessary. The work also dealt with such subjects as radio propagation, frequency planning and regulation, modulation schemes, antenna designs and teletraffic. The requirements of antenna system were considered vis-à-vis the terrain, topography and infrastructure. In sectored and large cells, directional antenna was proposed, whereas, in more populous, business areas, omnidirectional antenna was proposed. For the work, availability of land, infrastructure, and legal regulations were also considered. Availability of frequency band being greatly constrained, frequency reuse was given its due propriety. At the same time, care was taken to avoid interference owing to multiple transmissions. For this a careful combination power control and frequency planning was undertaken. 5. Limitations & Recommendations Designing a cellular mobile radio communication system for the city of Gold Coast spread over an area over 300 sq-km is a complex task and requires considerable resources. Needless to say that financial support to gather authentic data and undertake fool proof planning is essential. This is one of the biggest limitations of this work. It is recommended that financial assistance may be considered to be provided for undertaking works of this size and nature. 6. Conclusion and Future Work Cellular mobile radio system is an important and integral part of urban infrastructure. Apart from providing effective mobility to mass communication to a large number of business users, it also serves as the backbone of the urban social fabric. Therefore, planning and designing a cellular system is an important task. But for a city of the size of Gold Coast is a challenging task due to the typical topology, and traffic demand. Thus it requires expertise in many aspects such as radio propagation, frequency regulation and planning, modulation, antenna design, transmission planning, switching exchange, teletraffic and software design. Besides, other skills like human and economic factors are also required. The researcher in future wishes to undertake a full-fledged designing of a cellular system for a larger city with adequate support including professional software for doing delivering a fool proof design. Reference [1] Kunt Erik Walter and Per Hjalmar Lehne. www.telenor.com. [Online]. http://www.telenor.com/telektronikk/volumes/pdf/4.1995/Page_003-014.pdf [2] David W. Tipper, Chalermpol Charnsripinyo, and Hyundoo Shin. www.sis.pitt.edu. 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Mustafa, "Design of Cellular Mobile Radio System for Brisbane City and Suburbs," pp. 1-8, August 2009. [10] E.J Leonardo and Yocoub, "Cell coverage area using statistical methods," in Proceedings of the 7th IEEE Telecom. Conf. GLOBECOM’93, Houston, 93, pp. 1227-1231. [11] http://www.world-gazetteer.com. Australia: largest cities and towns and statistics of their population. [Online]. http://www.world-gazetteer.com/wg.php?x=&men=gcis&lng=en&des=wg&srt=npan&col=abcdefghinoq&msz=1500&geo=-24 [12] IEEE. IEEE Editorial Style Manual. [Online]. http://www.ieee.org/documents/stylemanual.pdf [13] www.alino.com. Radiofrequency Fields from Mobile Phone Technology. [Online]. http://www.alino.com/Info/RadiofrequencyFields/radiofrequencyfields.htm [14] (2009) api.ning.co. [Online]. http://api.ning.com/files/At5SNcF3nhDrjeLadNQMAeJdFT3Us1XUwGJE5ErPF2c_/andrew.jpg [15] people.seas.harvard.edu. people.seas.harvard.edu. 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